fix: move media server to 8093 — YOURLS/Apache owns 8092 on xNAS

The Unraid deployment's /media/api requests were answered by an
Apache/YOURLS stack squatting on 8092, surfacing as UI 503s; the CI
healthcheck (curl without -f) counted that 503 as healthy, so run #15
deployed green against a broken media server.

- media server port 8092 -> 8093 everywhere (server, MCP, agent, nginx,
  compose, CI, .env.example, docs, tests).
- healthchecks use curl -f in compose and rebuild.yml so HTTP error
  statuses fail deployment.
- tests default to the Unraid deployment (VOICE_WEB_URL=
  https://192.168.86.2:9178); token goes through nginx (loopback :8091
  is unreachable off-host); bind-mount-coupled tests skip on remote
  targets. AGENTS.md documents the convention and both failure modes.

web/media_server.py

@@ -49,7 +49,10 @@ JOBS_DIR = MEDIA_DIR / "jobs"
 LIBRARY_FILE = MEDIA_DIR / "library.json"
 COOKIES_FILE = os.environ.get("YTDLP_COOKIES", "")
 MEDIA_HOST = os.environ.get("MEDIA_SERVER_HOST", "127.0.0.1")
-MEDIA_PORT = int(os.environ.get("MEDIA_SERVER_PORT", "8092"))
+# 8093, NOT 8092: 8092 is occupied by an Apache/YOURLS stack on the Unraid
+# deployment host (xNAS) — a bind conflict there looks like a healthy
+# deployment because the healthcheck curl (without -f) accepts any response.
+MEDIA_PORT = int(os.environ.get("MEDIA_SERVER_PORT", "8093"))
 
 DOWNLOAD_TIMEOUT = int(os.environ.get("MEDIA_DOWNLOAD_TIMEOUT", "600"))
 SEARCH_TIMEOUT = int(os.environ.get("MEDIA_SEARCH_TIMEOUT", "45"))